1. Field of the Invention
The present invention relates to a transflective thin film transistor substrate of a liquid crystal display device, and more particularly, to a transflective thin film transistor substrate and method of fabricating the same that is adaptive for simplifying its process.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices are classified into two types: the transmissive type in which images are displayed using light from a backlight unit, and the reflective type in which images are displayed using an external light such as a natural light. While the transmissive type has a disadvantage in that the power consumption of the backlight unit is high, the reflective type depends on the external light so that it cannot display images in a dark environment.
In order to solve such a problem, a transflective LCD device is being researched and manufactured. For the transflective LCD, either the transmissive mode where a backlight unit is used or the reflective mode where an external light is used can be selected. The transflective LCD device operates in the reflective mode when an external light is sufficient and operates in the transmissive mode when an external light is not sufficient. Thus, the transflective LCD can reduce power consumption compared with the transmissive LCD device without being dependent upon an external light.
Referring to FIG. 1, a transflective LCD panel according to the related art includes a color filter substrate and a thin film transistor substrate which are bonded together with a liquid crystal layer (not shown) between the two substrates. A backlight unit 60 is arranged behind the thin film transistor substrate. Each pixel of the transflective LCD panel is divided into a reflective area where a reflective electrode 28 is formed, and a transmissive area where the reflective electrode 28 is not formed.
The color filter substrate includes a black matrix (not shown), a color filter 54 formed on an upper substrate 52, a common electrode 56 and an alignment film (not shown) formed thereover.
The thin film transistor substrate includes a gate line 4 and a data line (not shown) formed on a lower substrate 2 to define each pixel area, a thin film transistor connected to the gate line 4 and the data line, a pixel electrode 32 formed at the pixel area and connected to the thin film transistor, and the reflective electrode 28 formed at a reflection area of each pixel to overlap the pixel electrode.
The thin film transistor includes a gate electrode 6 connected to the gate line 4, a source electrode 16 connected to the data line, a drain electrode 18 facing the source electrode 16, an active layer 10 that overlaps the gate electrode 6 with a gate insulating film 8 therebetween to form a channel between the source and drain electrodes 16 and 18, and an ohmic contact layer 12 to form an ohmic contact between the active layer 10 and the source and drain electrodes 16 and 18. The thin film transistor responds to a scan signal of the gate line 4 to charge a video signal in the pixel electrode 32 via the data line.
The reflective electrode 28 reflects an external light that is incident through a color filter substrate, toward the color filter substrate. The surface of an organic film 24 formed under the reflective electrode 28 has an embossed shape. Thus, the reflective electrode 28 has also an embossed surface, following the contours of the organic film 24, thereby increasing the reflection efficiency of the reflective electrode 28 due to the dispersion effect of the embossed surface.
The pixel electrode 32 is connected to the drain electrode of the thin film transistor, and the pixel electrode 32 supplied with a pixel signal through the thin film transistor generates a potential difference with respect to the common electrode 56. The potential difference causes a liquid crystal having a dielectric anisotropy to rotate, thereby controlling the transmissivity of the light that passes through the liquid crystal layer in each of the reflection area and a transmission area.
A transmission hole 36 is formed in the relatively thick organic film 24 at the transmission area so that a length of the light path passing through the liquid crystal layer in the reflection area is substantially the same as a length of the light path in the transmission area. In other words, the length of the light path that an ambient light being incident to the reflection area (reflection light RL) travels, i.e., the liquid crystal layer, the reflective electrode 28 and the liquid crystal layer is substantially the same as the length of the light path that the transmission light TL of the backlight unit 60, which is incident to the transmission area, travels. Thus, the transmission efficiency of the reflection mode becomes the same as that of the transmission mode.
The thin film transistor substrate further includes a storage capacitor connected to the pixel electrode 32 in order to stably maintain the video signal supplied to the pixel electrode 32. The storage capacitor is comprised of an upper storage electrode 20 and the gate line 4 with the gate insulating film 8 therebetween. The storage capacitor further includes the ohmic contact layer 12 and the active layer 10 between the upper storage electrode 20 and the gate line 4.
The thin film transistor substrate further includes a first passivation film 22 between the thin film transistor and the organic film 24, a second passivation film 26 between the organic film 24 and the reflective electrode 28, and a third passivation film 30 between the reflective electrode 28 and the pixel electrode 32. Accordingly, first and second contact holes 34 and 38 penetrate the first to third passivation films 22, 26 and 30, the organic film 24 and the reflective electrode 28, so that the pixel electrode 32 is connected to the upper storage electrode 20.
Because forming such a transflective LCD panel requires a plurality of mask processes, its manufacturing process is complicated and manufacturing cost increases.
Hereinafter, a method of fabricating a transflective thin film transistor substrate according to the related art will be described in reference with FIGS. 2A to 2F.
Referring to FIG. 2A, a gate pattern including the gate line 4 and the gate electrode 6 is formed on the lower substrate 2 using a first mask. To do so, a gate metal layer is formed on the lower substrate 2 by a deposition method such as sputtering. Subsequently, the gate metal layer is patterned by a photolithography process using the first mask and an etching process, thereby forming the gate pattern including the gate line 4 and the gate electrode 8. The gate metal layer is a single layer or double layer structure of metal such as Al, Mo, Cr or the like.
Referring to FIG. 2B, the gate insulating film 8 is formed on the substrate 2 where the gate pattern is formed, and a source/drain pattern is formed on the gate insulating film 8 using a second mask, which includes the data line, the source electrode 16, the drain electrode 18 and the upper storage electrode 20. To do so, the gate insulating film 8, an amorphous silicon layer, an amorphous silicon layer doped with impurities doped thereto, and a source/drain metal layer are sequentially formed on the lower substrate 2 where the gate pattern is formed. The gate insulating film 8 is formed of an inorganic insulating material such as silicon oxide SiOx or silicon nitride SiNx, and the source/drain metal layer is a single-layer or double-layer structure of metal such as Al, Mo, Cr, or the like.
A photo-resist pattern is formed on top of the source/drain metal layer by a photolithography process using the second mask. In this case, a diffractive exposure mask having a diffractive exposure part at a channel portion of the thin film transistor is used as the second mask, thus the photo-resist pattern of the channel portion has a lower height than the source/drain pattern portion.
Subsequently, the source/drain metal layer is patterned by a wet etching process using the photo resist pattern to form the source/drain pattern that includes the data line, the source electrode 16, the drain electrode 18 integrated with the source electrode 16, and the upper storage electrode 20.
Then, the amorphous silicon layer doped with the impurities and the amorphous silicon layer are simultaneously patterned by a dry etching process using the same photo-resist pattern, thereby forming the ohmic contact layer 12 and the active layer 10.
After removing the photo resist pattern having a relatively low height at the channel portion by an ashing process, the source/drain pattern and the ohmic contact layer 12 of the channel portion are etched by a dry etching process. Accordingly, the active layer 10 of the channel portion is exposed to separate the source electrode 16 from the drain electrode 18.
Subsequently, the photo-resist pattern remaining on the source/drain pattern is removed by a strip process.
Referring to FIG. 2C, the first passivation film 22 is formed on the gate insulating film 8 where the source/drain pattern is formed, and an organic film 24 is formed on top thereof using a third mask. With the third mask, first and second contact holes 35 and 37 and the transmission hole 36 with an embossed surface are formed in the organic film 24.
To do so, the first passivation film 22 and the organic film 24 are sequentially formed on the gate insulating film 8 where the source/drain pattern is formed. The first passivation film 22 is formed of, for example, the same inorganic insulating material as the gate insulating film 8, and the organic film 24 is formed of a photosensitive organic material such as acrylic resin.
And then, the organic film 24 is patterned by a photolithography process using the third mask, thereby forming the first and second contact holes 35 and 37 and the transmission hole 36 in the organic film 24. The third mask has a structure where a shielding part and a diffractive exposure part are repeated except for the area corresponding to the transmission part of the pixel area. Thus, the organic film 24 is patterned to have a structure where a projected part corresponding to the shielding area of the third mask and a groove part corresponding to the diffractive exposure area are repeated. Subsequently, the organic film 24 having an embossed shape is cured.
Referring to FIG. 2D, the second passivation film 26 is formed on the organic film 24 that has the embossed shape, and the reflective electrode 28 is formed on top thereof using a fourth mask.
To do so, the second passivation film 26 and a reflective metal layer are deposited, following the contours of the embossed shape of the organic film 24. The second passivation film 26 is formed of, for example, the inorganic insulating material such as the first passivation film 22, and the reflective metal layer is formed of metal such as AlNd or the like, of which the reflectivity is high.
Subsequently, the reflective metal layer is patterned by a photolithography process using a fourth mask and an etching process to form the reflective electrode 28. the reflective electrode has openings at the transmission hole 36 and the first and second open holes 35 and 37 of the organic film 24 in each pixel area.
Referring to FIG. 2E, the third passivation film 30 covering the reflective electrode 28 is formed using a fifth mask. With the fifth mask, first and second contact holes 34 and 38 penetrating the first to third passivation films 22, 26 and 30 are formed.
To do so, the third passivation film 30 covering the reflective electrode 28 is formed on top of the reflective electrode 28. Then, the first and second contact holes 34 and 38 are formed by a photolithography process and an etching process using the fifth mask. Because the first and second contact holes 34 and 38 penetrate the first to third passivation films 22, 26 and 30, the first and second contact holes 34 and 38 expose the drain electrode 18 and the upper storage electrode 20. The third passivation film 30 is formed of, for example, the same inorganic insulating material as the second passivation film 26.
Referring to FIG. 2F, the pixel electrode 32 is formed on the third passivation film 30 using a sixth mask.
To do so, a transparent conductive layer is formed on the third passivation film 30 by a deposition method such as sputtering. Then, the transparent conductive layer is patterned by a photolithography process and an etching process using the sixth mask to form the pixel electrode 32 in each pixel area. The pixel electrode 32 is connected to the drain electrode 18 and the upper storage electrode 20 through the first and second contact holes 34 and 38. The transparent conductive layer is formed of, for example, indium-tin-oxide ITO.
As described above, the related art transflective thin film transistor substrate is fabricated using six masks so that its manufacturing process is complicated. Further, a sufficient process margin of the first and second contact holes 34, 38 should be secured in order for the pixel electrode 32 to be connected to the drain electrode 18 and the upper storage electrode 20, thereby reducing the aperture ratio of the transmission area.